So we all know that interstellar starships that travel at high relativistic speeds, like 50% c or above, would be very susceptible to getting destroyed just by a grain of salt hitting it during travel. We know we can stop this from happening by using a physical barrier or the use of plasma shields.

But I was just wondering how about solar sailed ships? The ones whose diameters outsize the main ship in of itself. How can they be protected during a travel in interstellar space?

And I know traditional solar sails won't travel that fast but what about photon laser sailed ships that can theoretically allow travel at 30% to 50% lightspeed? How can it be shielded from such obstacles?

I'm not really a physics type of guy so I just wanna see what you guys have to say.

$\begingroup$If a solar sail had small holes .. it wouldn't matter.$\endgroup$
– FattieJul 1 at 10:07

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$\begingroup$"[...]using a physical barrier[...]" so what exactly is the difference between your physical barrier and say, your ship's hull? The nuclear explosion would just happen a few meters in front of your ship. I would highly advise against this.$\endgroup$
– infinitezeroJul 2 at 12:06

$\begingroup$Lightsail 2 does not have anything to prevent holes from happening. What it does have instead are some seams that will limit hole growth. We shall see if that works. But note that even at 50%c, you can't realistically travel between stars - even going to some remote corners of this solar system will take quite a while.$\endgroup$
– Zizy ArcherJul 3 at 13:11

10 Answers
10

Any impact with dust or sub-dust size object (which is more likely to happen) will pierce a hole through the sail (and leave a cloud of plasma behind it).

Just "patch" it using the nanobots and keep traveling.

For larger objects between the size of dust and meter sized asteroids, I would better opt for selectively close the sail upon detection, since damage might be too extensive to repair. (how you detect it in time is another story)

For even larger objects, well, sorry, no much to do once you are heading at 0.5c on such a bulky obstacle.

A tiny piece of space debris is dangerous to the ship, as it may hit people, mechanisms, or fuel. But the damage to the hull itself would be negligible. It could simply be patched. The hull is the least of the concerns there. Solar sails typically don't contain people, mechanisms, or fuel that could be damaged.

A solar sail with a tiny hole in it could also be patched, whether with nanobots or with a regular robot and a patching kit. You could even send a human out to do it, although the human would then be effectively unprotected from more space debris.

For that matter, for some substances, the hole itself might be self-patching. Consider an opaque form of ice, say frozen mercury. The impact would liquefy the mercury, which might then close the hole after it. It would require some experimentation, but it seems like there should be some form of matter that would work like that. Mercury may not be that form of matter. It's just a possibility that came to mind.

Remember that at space temperatures, even things that are normally gases would be solid. Perhaps not hydrogen, but something should have a triple point and liquid cohesion that would work.

Beyond that, the hole would be tiny relative to the size of the sail. A big hole would add torque. But for the typical worry, the hole would be so tiny as to not be noticeable. The far greater cause of torque would be for the light source to hit unevenly. So if the sail can handle an imperfect light source, it should be able to handle tiny holes.

It would also be common to spin the ship to get an artificial gravity effect. This might offset the torque issue by constantly changing where the torque is.

$\begingroup$"Remember that at space temperatures" -- Space has a temperature in the same way that "off" is a radio station. That is, no heat will convect or conduct away. Your solar sail won't freeze. In fact, since it's catching as much sunlight as possible, it'll probably be one of the hotter parts of your ship. To freeze your sail, you'll need active cooling (heavy, energy hungry -- the opposite of what you want a solar sail for) and will need to pump that heat to radiators. At that point, you have a large electric generator or huge solar power array, so you might as well have an engine.$\endgroup$
– GhedipunkJul 2 at 21:22

The word here is redundancy. Your sail will take damage, that much is pretty unavoidable, but the great thing about solar sails (unlike wind sails) is that the continuity of the sail isn’t important: only total surface area is.

So break up the sail into a cluster of smaller sails. If one gets damaged then unclip it, replace it with a sail you had folded in storage, and repair the damaged sail. If you make the sails manoeuvrable you can shift the distribution of sails to keep thrust even while the damaged sail is replaced, and even selectively open holes in the cluster to allow detected obstacles straight through.

This will be a tricky engineering challenge. Dynamically controlling the attitude of the sails, the tension on whatever you’re using to tether them to your vessel and also managing the distances between the sails is non-trivial to say the least. Photonic laser thrusters (basically satellites pointing lasers at each other) could help in formation keeping, as could angling the sails (though that would reduce overall thrust) but it’s not simple, to say the least.

A solar sail is certainly fragile, and can easily by punctured. However, a puncture won't reduce the total area of the sail by very much, so it won't reduce the effectiveness of the sail by very much either. The real danger is tearing of the sail, because that can greatly reduce not just the area of the sail but also your ability to control how the sail is pointed. Therefore the sail needs to be designed with ripstops in it. A ripstop is any feature which tends to prevent a tear from propagating any further. You can segment the sail into smaller panels (a tear can't propagate across the gap between panels), or you can strengthen the material in several ways.

The bulk of the sail may be extremely thin (a few dozen microns for the designs NASA has studied) if it's criss-crossed by thicker material that is less likely to tear. Most of the designs studied by NASA were plastic (Kapton is light weight, strong, and radiation resistant enough not to be damaged by the Sun's strong UV radiation too quickly) with metal vapor deposited on one or both sides (to make it shiny). Most of NASA's designs then had extra strips of tape glued to the surface of the sail to form the ripstops. Other designs imagined making the sails out of smaller panels that were glued together with strips that would also form ripstops. Other designs had sail panels that weren't attached to each other at all, but were instead connected to the framework of the sail in a manner very similar to how a sail is attached to the rigging of a boat. They found of course that while smaller panels were more tear-resistant, the weight of the framework would increase quite rapidly as the panels became smaller.

Most designs had a combination of these features; the square ones in the pictures you found would most likely be four triangles rigged to a framework with four long spars along the diagonals. (Given the size of the sail, it's quite possible that the gap between panels would be invisible; even so that picture does show four panels each with the texture running in a different direction.)

If you'd like a reference where you can read about all of these and many other important details, then I recommend "Space Sailing" by Jerome L Wright. He covers not just the design of a solar sail, but how you steer one, how you plan a mission using a sail, how the control system needs to run continuously in order to successfully account for the inevitable deviations from your planned route, etc.

What @L.Dutch said OR realise that you are going to take irreparable damage and design a margin of safety into the sail. This will mean a "waffle" design of reinforced cells so that punctures don't turn into tears and a sail that is a certain percentage over-sized for the trip in question. This may be up to several times the size it needs to be to supply the design thrust initially, the journey may start with sails partially furled to compensate for this. Per L.Dutch's comment such a sail may also require a mechanism that allows it to be furled or tilted selectively as damage occurs in order to keep the sail in trim and properly aligned with the light source it is utilising. Such sails will be designed to be either refurbished after every trip or simply dumped and replaced on regular routes with the remains of old sails being fed to nano-fabricators as raw material.

$\begingroup$Side note: if you accept holes in the sail, you are going to get an induced torque on it, and as a result the sail will start spinning, thus losing alignment with the light source.$\endgroup$
– L.Dutch♦Jun 30 at 11:14

$\begingroup$@L.Dutch So you'd also need a way to keep such a sail in trim, thanks I hadn't thought of that.$\endgroup$
– AshJun 30 at 11:15

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$\begingroup$The ship and sail will likely be spinning on its long axis anyway, so any induced torque will be evenly distributed, and only count as a loss of thrust. It's hard to imagine a few pinholes in a sail mattering much though$\endgroup$
– InnovineJun 30 at 11:31

$\begingroup$@innovine torque on a spinning object doesn't just average out. There will be nasty complicated gyroscopic effects.$\endgroup$
– craqJun 30 at 17:55

Point-defense and Origami Sails

Keeping a laser on target over space distances is really hard, requires a lot of energy and communication lag can lead to horrible accidents. So you don't do it for the entire journey unless there are laser highway pushing stations along the way. There will be an acceleration and deceleration window of several hundred to a few thousand AU to accelerate and decelerate the laser sailor. Outside of these windows, you don't need the sail, so you use a design which can be reeled in like the sails of ships in a huge origami exercise. Carbon nanotubes should give you a yarn light and strong enough to do the job. The reel in doesn't need to be quick either, it can happen over several hours or even weeks. The sail is stored inside the debris shields protective shadow for most of the journey until it is redeployed.

As for the protection of the sail and vessel a debris shield is a good start, yet what kind of shield would actually be used would really depend on the author's preferences and the technology of the setting, as plasma-shields, Whipple-shields, big-slightly-rounded-and-cool-looking-shields and pointed dagger-shields all have their own advantages and drawbacks. But even better than being able to take a hit is not getting hit at all. No shield in the world will help against a kilometer big space rock, evading it with maneuvering thrusters is the way to go. Another way of getting rid of space debris is point defense. Carrying huge telescopes may be made of repurposed sail material will allow the vessel to get the mass, vector, and position of incoming debris. An array of lasers can then be used to shove the rocks of a collision course or to vaporize them. This can be used to reduce the strain on the shield and to keep the sails save during the acceleration and deployment phases. If the sails are kept out in the open having dedicated holes in the sails to herd the debris through or slowly moving the sail parallel to the vector of travel to reduce the cross-section the point defense system needs to protect might be options as well.

They used the sail itself as a "weapon" to clear debris out of the way.

Radar watch to detect rocks, then use the sail itself as a reflector to aim and concentrate a beam of sunlight to either vaporize small stuff or to use ablation as a form of propulsion on larger rocks to make them move out of your way.

$\begingroup$This is definitely a creative solution. How practical it might be to actually implement, I'm not certain.$\endgroup$
– Cliff HallJul 1 at 16:19

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$\begingroup$@CliffHall: I'm not sure how practical it would be, either. It is mentioned in passing in the book, and the humans discussing it pretty much just assumed that's how it worked - they didn't go into great detail. It is used as part of the plot device to explain how the Moties on the light probe destroyed evidence of the existence of the warrior caste Moties that had been on board the probe.$\endgroup$
– JREJul 1 at 16:24

$\begingroup$I was going to suggest SPACE LASERS but I like this even better. Especially if for stuff overtaking you from behind, because then the plasma you create can still confer its kinetic energy when it hits you, gently.$\endgroup$
– WillkJul 2 at 19:05

The point of a solar sail is to be as light as possible so that the force exerted upon it by photons is enough to accelerate it.

Shields are heavy and make acceleration much more difficult.

Equipment to blast oncoming debris out of the way is heavy and requires more power than you likely have being solar-powered.

Nanobots on a solar sail? What are they making the repair stuff out of and what factory is continuously creating the new nanobots since they're molecular machines that can't be repaired without nanobots to repair the nanobots. If you have an effective nanobot factory, you probably have a better propulsion method than a solar sail.

I'd say, make it be as big as possible so that the smaller holes punched through it don't matter much. And if something really big is detected, then maybe fold the sail up in until it passes, then unfurl again.

Basically, at high velocity, you aren't going to be able to absorb an impact, so you don't even try to. Instead, rather than trying to armor things, you deploy a thin layer of material out in front of your spacecraft so that an impactor vaporizes itself and a small amount of shield material on impact, with the energy release travelling outwards in a roughly spherical fashion. As a result, it winds up spread much more widely over the hull structure (or sail structure) behind it. With a sufficient distance between them, you can reduce the amount of energy a given surface of hull or sail experiences from an impact below the amount of pressure needed to cause it to fail.

$\begingroup$The big issue with whipple shilds is that they will only take out one incoming projectile on the ame spot and that they will double the mass of the sail.$\endgroup$
– TheDyingOfLightJun 30 at 17:40

A grain of sand won't destroy a solar sail. It'll just punch a tiny hole in it.

Small scale experiments must be performed to determine exactly how many holes per cm^2 per unit of time will be created, and how much that reduces the efficiency.

Only then should we decide what kind of shields we should use. (It might just be cheaper (that's shorthand for "most efficient") segment the sail (think "ripstop nylon") and pack a bunch of spare sail segments.

$\begingroup$@TKK the abstract (which is all I have access to) of that article is about large meteors hitting the atmosphere. If you have access to the whole thing, I'd love to see some graphs.$\endgroup$
– RonJohnJul 2 at 21:05

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$\begingroup$From a cursory read, that paper concerns the fusion which occurs in the sustained high-pressure hydrogen-rich plasma sheath of a comet hurtling through the atmosphere. The infinitesimal fraction of time a dust grain would take to punch through a solar sail would not support such conditions. A grain of sand at 0.5 c would still cause as much damage as about 200 g of TNT, though, simply by calculating its kinetic energy.$\endgroup$
– BBeastJul 3 at 12:01

$\begingroup$@BBeast thanks, that's what I expected. What direction will that energy dissipate (equally in all directions, or in a cone along the axis of travel of the grain of sand)?$\endgroup$
– RonJohnJul 3 at 12:19

$\begingroup$@RonJohn no idea. My best guess would be to assume the grain would transfer a substantial amount of kinetic energy to the material it impacts, blasting out a cone of plasma (analogously to a bullet punching through a metal sheet), but that's only a guess. Whatever happens, you would likely get a hole noticeably bigger than the grain of sand itself from a relativistic impact. Still tiny on the scale of a solar sail, though.$\endgroup$
– BBeastJul 4 at 0:20